Yes, gasses will mix totally, but have you seen the trail of compressed liquid N2 where it falls to the floor prior to mixing? I have seen a room where the floor area becomes a highly concentrated N2 to air mix and the O2 monitor will sound the alarm.
The cold stream of N2 falls to the floor because its density is greater than the ambient air and does not mix well until it warms. Soon, when the N2 does warm up and mixes with the air, the total N2 concentration mix could be well into the danger level for breathing. It does not take long for a person to pass out in that type of atmosphere. That is the danger if the only O2 monitor is at a higher than head level. In the lab where I worked, we had a monitor near the floor level and one at a higher level. This would sound the alarm before a person would be asphyxiated when entering.
Even with a normal HVAC system air flow and high exhaust vent, the density of the cold N2 still would collect on the floor and could be a danger. It is better if an exhaust is near the floor to collect it. This is especially needed if the room is dedicated to liquid N2 dewars for cold storage.
I see this as a part of training needed when handling compressed gasses, and especially if they are the liquid compressed gasses. The designed HVAC system needs to be part of the equation to properly handle this danger.
Ken (retired ex CIH)
I think there is a general misunderstanding about gas phase miscibility among many. Substances which exist in the vapor phase under ambient (existing) conditions are infinitely miscible. They will eventually form a complete gaseous solution (vial convectional mixing and Brownian motion) and will not separate or "settle out" after mixing unless they react to form a compound that is no longer a gas under the existing temperature and pressure (precipitation) or unless the existing temperature or pressure are changed, for example, in fractional distillation. One or more gas species in a gaseous solution could also preferentially adsorb onto a solid or liquid surface but I do not think this is something most people consider when dealing with gas releases. I have read that there are conditions of temperature and pressure under which gases may become immiscible but it does not appear to me that these would exist outside of an atmospheric chamber.
If someone can point me to a text or other source of information to the contrary I would greatly appreciate it.
On Sat, Dec 26, 2015 at 8:42 AM, Ralph Stuart <rstuartcih**At_Symbol_Here**me.com> wrote:
> It also makes the point I have trouble getting people to understand--that heavier than air does not mean put the ventilation exhaust at the floor! Once airborne and mixed well with air, vapors will disperse and will not resettle quickly. It's why heavy chlorinated solvents like the Freons end up in the stratosphere.
This is an important point. I heard a recent discussion with a PhD chemist who intuitively assumed that acetone and chloroform would move differently in the lab atmosphere. After all, he said, acetone is lighter than air (it isn't - it's vapor density is around 2) and chloroform isn't. My guess is that this sort of ad hoc reasoning is common among chemists who haven't had the occasion to think through macro properties of the chemicals they work with.
Ralph Stuart, CIH
Allen Niemi, PhD
Occupational Safety and Health Services
Room 322 Lakeshore Center
Michigan Technological University
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